This invention relates to phased-array ultrasound imaging systems and, more particularly, to methods and apparatus for facilitating communication between an ultrasonic transducer probe and an electronic console that performs beamforming and signal processing.
Ultrasonic or acoustic sensing techniques have earned a pre-eminent position in a variety of fields including medicine, nondestructive testing and process monitoring, geophysics, and sonar surveillance. For several decades, applications have exploited the relatively low expense, reliability, and enormous versatility of this modality. A strong theoretical understanding of ultrasonics has been developed in parallel with this practical knowledge, so that improved performance has steadily continued. Much of today""s research and development is aimed at increasing the number of elements in an ultrasonic array, decreasing the size of the elements, or achieving both simultaneously. The resulting arrays would provide improved spatial or temporal resolution either by using higher frequencies, or by using true two-dimensional arrays for volumetric imaging. However, such advances present formidable technical challenges. Two major obstacles are element impedance and fabrication issues, and issues concerned with cabling between the sensor head and the electronic console.
Most ultrasonic transducers rely on the piezoelectric effect to detect and generate acoustic waves. The design and fabrication of piezoelectric elements remains as much an art as a science, and proves increasingly difficult as element size is decreased or element number increased. Difficulties are in part mechanical: the actual construction and handling of many extremely small components, the fabrication and xe2x80x9cdicingxe2x80x9d of multi-element arrays, reproducibility, and yield. Another major concern is electrical: as an element decreases in size, its impedance increases. Impedance matching, critical to signal sensitivity, presents additional complications. In particular, as the element impedance increases relative to the combined impedances of the coaxial line and the receiver circuit, less signal reaches the receiver circuit. Thus, for a given piezoelectric material, a reduction in element size is accompanied by a reduction in signal sensitivity.
The electrical cable bundle linking an ultrasonic array and its electronics also presents problems. Proper shielding is vital, since the cables are a major noise source. Cable length is restricted by the wire impedance relative to the element impedance. Furthermore, fabrication becomes more difficult as the number of array elements, and consequently the number of connecting wires, is increased. To avoid fabricating a hopelessly bulky and unmanageable cable, manufacturers must continually decrease the size of their coaxial wires. Although present technology can enable about 100 coaxial lines to fit into a narrow (a few millimeters in diameter) cable, cable size reductions cannot be continued indefinitely; at such small wire diameters, DC (direct current) resistance becomes significantly high. Additional practical difficulties are presented simply in using extremely small wires, for instance, in wire bonding or soldering.
In response to these challenges, optic, instead of electronic, control of piezoelectric elements has been proposed. Acquisition and control opto-electronics could be coupled to a transducer head via a fiber optic bundle; communication with the compact head would be by optical fibers. With present fiber optic technology, enough fibers for a 100xc3x9710 element array could fit inside a cable only a few millimeters in diameter. A thinner, more flexible cable of virtually any length offers added operator convenience, especially for medical use. Medical implementations such as ultrasonic catheters and endoscopes could similarly benefit. Radioactive or other harsh environments could be inspected remotely, without damage to sensitive electronics. Ultrasonic evaluation of large, complex, and limited-access components, such as long tubes, bores, or piping, could be performed more easily. In addition, optical methods of communicating between a piezoelectric transducer array and an electronic console could enable new applications that are not feasible with present technology, for instance in remote sensing or xe2x80x9csmart structuresxe2x80x9d.
To facilitate optical communication between a transducer probe and an electronic console, it has been proposed to detect ultrasound using a micro-cavity laser, which requires only an optical connection to the transducer probe. The proposed prior art method uses a monolithic laser cavity, such as a microchip laser, in place of a piezoelectric crystal. In its simplest form, the microchip consists of small xe2x80x9cchipxe2x80x9d (of area ≈1 mm2) of a lasing medium, which is cut and polished flat on two parallel sides. By depositing dielectric mirror coatings on these flat sides, a laser cavity is defined. Lasing is accomplished by optically pumping with a separate laser tuned to an absorption band of the microchip. The materials that have seen the most development as microchip laser media include neodymium-doped crystals such as NdxY3xe2x88x92xAl5O12 (Nd:YAG) and NdxY1xe2x88x92xVO4 (Nd:YVO4). These crystals have exhibited quite efficient CW lasing (≈30% optical efficiency) when pumped either by a Ti:sapphire laser or a diode laser.
The proposed prior art method relies on the fact that when the cavity length (L) of the laser is changed, the optical frequency (xcexdo) emitted by the laser changes such that the fractional length change is equal to the fractional frequency change as set forth in the following equation:                                           Δ            ⁢                          xe2x80x83                        ⁢            v                                v            o                          =                              Δ            ⁢                          xe2x80x83                        ⁢            L                    L                                    (        1        )            
When the laser cavity is placed in a time-varying acoustic field, the cavity length of the laser should be modulated with the same time dependence as the acoustic field and with an amplitude related to the amplitude of the acoustic field. As a result, Eq. (1) shows that the optical energy output of the laser should be frequency modulated with a time dependence and amplitude determined by the respective time dependence and amplitude of the acoustic field. The frequency-modulated optical energy can then be demodulated and converted into an electrical output signal remotely from the microchip for signal analysis. The original time-varying acoustic field can be recovered by frequency demodulating the optical signal using a slope filter.
In order for the aforesaid detection method to be advantageous, the laser detector must be of small size (e.g., active area  less than 1 mm2) and free from any electrical connections. Microchip laser technology satisfies these requirements. A microchip laser comprises a xe2x80x9cchipxe2x80x9d of a lasing medium such as neodymium-doped yttrium aluminum garnet (Nd:YAG), fabricated with dielectric mirror coatings on two ends so that it can be optically pumped. When pumped by a wavelength corresponding to an absorption band, the lasing process can be accomplished with a remotely situated, low-power laser delivered through an optical fiber. The mirror coatings can be arranged so that the microchip laser output energy returns through the same fiber. Since the return light is a different wavelength from the pump light, it can be separated with a wavelength demultiplexer and then frequency demodulated to extract the optical signal component determined by the time dependence and amplitude of the acoustic field.
Equation (1) shows that the frequency shift experienced by the microchip laser output energy depends on the macroscopic strain (xcex94L/L) experienced by the microchip. The macroscopic strain is related to the microscopic strain S(x,t) by the following equation:                                           Δ            ⁢                          xe2x80x83                        ⁢                          L              ⁡                              (                t                )                                              L                =                              1            L                    ⁢                                    ∫              o              L                        ⁢                                          S                ⁡                                  (                                      x                    ,                    t                                    )                                            ⁢                              xe2x80x83                            ⁢                                                ⅆ                  x                                .                                                                        (        2        )            
where x represents position along the thickness of the micro-cavity laser and t is time. Given any arbitrary acoustic disturbance in the microchip, the change in lasing frequency can be calculated using Eqs. (1) and (2). The microscopic strain that develops in the microchip in response to an incident acoustic wave depends sensitively on the ratio of the microchip cavity length to the acoustic wavelength, the acoustic impedance of the microchip relative to the medium in which it is embedded, and the acoustic impedence of any other structures such as acoustic matching layers attached to the microchip.
Although micro-cavity lasers have been demonstrated to detect ultrasound at frequencies and intensities compatible with medical imaging, it is difficult to construct a suitable transmitter in the same structure as the laser detector. There is a need for a structure which overcomes this difficulty.
The invention overcomes the foregoing difficulty by incorporating micro-cavity lasers in a piezocomposite material of an ultrasonic transducer. As used herein, the term xe2x80x9cpiezocompositexe2x80x9d refers to a combination of a piezoelectric material (e.g., piezoelectric ceramic or single-crystal piezoelectric material) and a non-piezoelectric polymer to form a new piezoelectric material.
Piezocomposite materials were introduced in the past to reduce the lateral mechanical coupling inherent in bulk piezoelectric ceramic. In accordance with the conventional technique, isolated long, thin piezoceramic rods (e.g., PZT) are interspersed in and held parallel to each other by a passive polymer matrix. As long as the spacing is small compared to the wavelength, the transducer element will vibrate uniformly, as a whole, with the elastic properties of the effective medium formed by the piezoceramic and the polymer. The resulting elements are effective as both transmitters and receivers of ultrasound.
Also well known in the prior art are micro-cavity lasers used as detectors of ultrasound. The cavity responds to acoustic pressure changes by changing its length. As a laser cavity, the resonant frequency of the cavity, and hence the frequency of the output beam, is very sensitive to the length of the cavity. Since the optical beam can be examined with high-finesse optical components, it is possible to detect small frequency shifts with great accuracy without direct electrical connections. The output optical energy of the micro-cavity laser is simply coupled to a fiber optic cable and transmitted to the console, where the optical system frequency modulator detects the laser output optical energy.
The materials used for micro-cavity lasers are typically transition metal and rare-earth doped glasses. These materials tend to be rather dense and have specific acoustic impedances in the same range as commonly used piezoceramic materials like PZT. These acoustic impedances are often in the range of 25 to 40 MRayls (1 Rayleigh or Rayl is 1 kilogram/square meter/second). Since these elastic properties are comparable to the piezoelectric material properties, it is possible to replace some of the rods in a piezoelectric-polymer composite with micro-cavity laser rods (i.e., micro-cavity lasers in the shape of rods) without substantially changing the modes of vibration of the resulting structure. The resulting assembly is capable of transmitting and receiving ultrasound energy like a conventional composite transducer. By using the micro-cavity laser segments, received ultrasound can be sensed without requiring electrical connections to convey the acquired data from the array to the electronic console for beamforming and signal processing.
In another aspect, the invention is directed to the foregoing composite structure, comprising piezoelectric (e.g., ceramic or single crystal) rods and micro-cavity laser rods embedded in a passive polymer matrix, and a method for manufacturing such a composite structure.
In another aspect, the invention is directed to facilitating communication between a multi-element ultrasonic transducer probe and an electronic console, e.g., a console comprising an electronic beamformer. The probe and the console are coupled by optical means, e.g., optical fibers. Each transducer element in the probe comprises a polymer matrix having rods of piezoelectric material and micro-cavity laser rods embedded therein.
In accordance with a preferred embodiment of the invention, the probe receives high-voltage electrical energy from a power source via an electrical cable and receives a multiplicity of optical beamforming signals from a transmit beamformer via a respective multiplicity of optical fibers. The electrical power is distributed to the transducer elements via a multiplicity of optically controlled switches which receive the optical beamforming signals. As a result, the piezoelectric material of respective transducer elements is activated to transmit an ultrasonic beam.
In accordance with another preferred embodiment of the invention, the ultrasonic echoes are detected by micro-cavity lasers incorporated in the transducer elements. The resulting optical output signals of the micro-cavity lasers are frequency demodulated to isolate the component corresponding to the received acoustic signal. The frequency-demodulated optical signals are then sent to the electronic receive beamformer. Preferably, the frequency demodulators are located within the console and are coupled to the probe via optical fibers.